Heat treatment of machined parts manufactured from nickel chromium alloys



Patented May 4, 1954 UNITED STATE i ATENT OFFICE HEAT TREATMENT OF Il/IACHINED PARTS MANUFACTURED FROM NICKEL CHRO- MIUM ALLOYS No Drawing. Application December 20, 1951,

Serial No. 262,658

Claims priority, application Great Britain February 17, 1949 8 Claims. 1

This application is acontinuation-in-part of our co-pending application Ser. No. 143,8l3,'filed February 11, 1950, now abandoned, which was based on our British application No. 4389/49, filed in Great Britain February 17, 1949. Certain added disclosure which appears herein and does not appear in Ser. No. 143,813 is based on our British application No. 31,676/50, filed in Great Britain December 30, 1950, for patent of addition to No. 4,389/49.

This invention relates to processes for the man ufacture of engineering parts from heat resisting alloys of the type which are commonly known as nickel-chromium alloys, and are capable of being hardened by precipitation heat treatment and containing one or more elements referred to as hardening elements.

The alloys to which this invention is'more particularly applicable are those falling within the following general description, namely: Alloys with a base (constituting at least 50 per cent of the alloy) of nickel or nickel plus cobalt (the cobalt not exceeding 40 per cent of the alloy), which contain chromium up to 45 per centof the alloy and which are capable of being'hardened by precipitation heat treatment since they contain one or more of the elements molybdenum, tungsten, iron, manganese, aluminum, vanadium, titanium, niobium, silicon and carbon and in which the carbon does not exceed one per cent neither the titanium nor the silicon exceeds 5 per cent and no one of the others exceeds 25 per cent and the total of all these elements does not exceed 0 per cent. Apart fromthe abovementioned elements and minor impurities the alloys will not contain other elements except that they may contain minor quantities of nitrogen, magnesium, copper, calcium and rare earth elements (including mischunetal) no one being present in an amount of more than '1 per cent and the whole not exceeding 3'per cent. These alloys are hereinafter called nickel-chromium alloys of the class described.

A number of alloys falling within the above general description have heretofore been developed, and one example thereof, described and claimed in our co-pending application Ser. No. 94,816, filed May 23, 1949, may be described in general terms as follows:

2 Cobalt exceeding 10 but not exceeding 40 per cent; Chromium from 10 to per cent; Molybdenum from 2 to 18 percent; Aluminium from 0.2 to 8.6 per-cent; Titanium from 0.2 to 4.4 per cent; Balance substantially nickel;

And in which the figure arrived at by adding up the molybdenum percentage, twice the aluminium percentage and four times the titanium percentage always-lies between 16 and 20. A nickel-chromium alloy of the kind last-mentioned is hereinafter referred to as a balance factor nickel-chromium alloy and a specific illustration thereof isas follows:

Per cent Chromium 20 Cobalt 20 Molybdenum '6 Aluminium 1 Titanium 2.5

Nickel the balance.

In .additionthere are other types of nickelchromium alloys of the class described, one of them being an alloy having the following composition:

Per cent Chromium .20

Titanium 2.4

Aluminium 0.5

Nickel substantially'the balance.

Nickel-chromium alloys of the class described, including the above examples, have found their typical use in the manufacture of parts of internal combustion gasturbines, for example, turbine blades. In such partsthe methods of manufacture must, among other things, be such as to achieve hardness, high creep strength and resistance to fatigue.

In the manufacture of finished pieces from cast billets of nickel-chromium alloys of the .class described, ithas been customary to employ two successive heat treatments. The first of these, ranging from 5-to=10 hours at 1050 to 1200 0.. is known as the solution-heat treatment. The second, ranging-from lil-to 20 hours at 700 :to 800 0., is'known as the-precipitation hardening heat treatment. These heat treatments have been directed in the main to imparting hardness and high creep strength to the alloy, and in typical practice hot-working such as forging, rolling or stamping of the cast billet is carried forward to the point at which the maximum cross-sectional reduction intended to be achieved by hot-working is completed at high temperatures on the order of 1050 to 1200 C., and thereafter the piece is subjected to the abovementioned solution heat treatment and precipitation hardening heat treatment, in the order named. Since, however, some parts must be machined to very close tolerances beyond any range of accuracy which is possible with hot-working, as for example in the case of turbine blades for internal combustion gas turbines, machining must follow the completion of hot-working. In ordinary practice machining follows solution heat treatment, and may precede or follow precipitation hardening heat treatment. the words machining and machined in this specification and the subjoined claims in a general and not a specific sense to refer to all of the known processes of cold-working a metal wherein there is a physical removal of material from the surface of the part being machined, such without limitation as grinding, drilling, polishing and milling, as distinguished from processes of cold-working a metal in which there is no physical removal of material but merely displacement, such for example as cold forging, rolling, drawing, spinning and bending.

Heretofore, difiiculty has been encountered in the case of machined pieces made from nickelchromium alloys of the class described, especially the thin trailing edges of turbine blades. Despite the utmost efforts at reliable control of the operating conditions, it has been found that such machined pieces when put in service will in time exhibit surface creep cracks in the region of the machined surfaces. While these cracks do not propagate in such a manner as to cause a failure under tensile load, they appear to initiate by localization of stress a failure due to fatigue.

Observation of this phenomenon has led us to make comparative creep strength tests on coldworked and unworked pieces of the same alloy and of substantially the same section thickness which had been heat treated in identically the same manner. In these tests the alloy under observation was that whose composition is given above as the specific illustration of a balance factor nickel-chromium alloy. The results are set forth in Table A below:

TABLE A Treatment Hours Cold Work, percent Surface Hardness Frac- 1 Time, tum

Time, h

Nil

1 Under stress of 33,600 lbs/in. at 815 C.

We use The tests shown in Table A have established that the cold-working of a specimen of nickelchromium of the class described produces the effect of materially reducing the creep strength of the metal at the machined surface. Tests have also shown that cold-working hardens the machined surface, the surface hardness rising from 270-300 to around 450-500, on the Diamond Pyramid Hardness Test scale. Lattice distortion X-ray observation methods indicate that the hardened skin has a depth of from about .014 to .016 inch and We believe that this skin is the region of the metal in which the reduction in creep strength caused by machining chiefly occurs. Such defect may be serious in parts of thin section, as for example, the thin trailing edges of turbine blades.

The present invention has for an object to restore the creep strength adjacent the machined surface of a piece made of nickel-chromium alloy of the class described which said creep strength has been reduced as a result of machining.

According to the present invention, the loss of creep strength in the surface of a machined piece of nickel-chromium alloy of the class described, created by machining, is alleviated and the full creep strength of an unmachined piece is recovered by the use, among other heat treatment steps, of a short time heat treatment which follows machining and normally precedes the final precipitation hardening heat treatment. In this short time heat treatment the piece, or at least the metal to a depth of about .016 inch beneath a machined surface of the piece, is heated for a length of time which, in the case of induction heating, may be as little as 30 seconds and not over an hour and, in the case of furnace heating. may be from 3 minutes to 1 hour, and during this time the piece or the heated portion thereof is held Within a temperature range which, generally speaking, is approximately from 850 to 1100 C. for nickel-chromium alloys of the class described, and which preferably is within the range 850 to 1000 C. for some kinds of these alloys and within the range 950 to 1100 C. for other kinds thereof as hereinafter shown. Ordinarily, the short time heat treatment as herein described will be carried out in sequence with other steps of the manufacturing process.

Example 1 By way of illustration, the following is representative of one sequence of steps which includes the process of the present invention:

(a) Forging a cast billet of a nickel-chromium alloy of the class described at a temperature in the general range l050 to 1200 C., the reduction in cross-sectional area of the piece being brought substantially to completion at this stage, and the piece being thereby readied for machining to final shape and tolerances;

(1)) Giving to the piece, as thus hot-worked, a solution heat treatment of 5 to 10 hours at 1050 to 1200 C.;

(c) Machining the forged piece to a finished size;

(01) Giving to the piece a short time heat treatment of 10 minutes to 1 hour in a furnace, or electrical induction heating of the machined surfaces for 30 seconds or more but not above 1 hour, at 850 to 1050 C.; and

(e) Giving to the piece a precipitation hardening heat treatment of 10 to 20 hours at 700 to 800 C.

As a specific illustration of the above-we have subjected a thin section piece having thexfollowing composition 1 this being a balance factor nickel-chromium alloy, to the treatments indicated in Table Bas follows:

subjecting the piece to solution heat treatment forf5='to.10 hours at 1050 to 1200 C,, then carrying out the balance of the hot-work shaping within thesametemperature range substantially to completion of the reduction .thereby intended, then subjecting the piece to a grain refining heat treatment of 5 minutes to 1 hour at'850fC. to 1050* C., and finally giving the piece a precipitation hardening heat treatment of 10 to-'hours at'700- to 800 C. This procedure has, among its objects, to produce in the body of the metal a fine grain structure at the same time that high creep strength is developed therein.

1 Under stress of 33,000 lbs./ in. at 815.C.

The results of Test No. 1 indicate that, if the piece is not machined, it has a satisfactoryhigh creep life, in this case 9'7 hours to fracture. When, however, the piece is subjected to substantial cold work producing a hardness equivalent to that produced by machining (Test No. 2) the creep life of the piece drops materially, despite application of the precipitation hardening heat treatment of 700 C. for 16 hours.

In Tests Nos. 3 and 4, however, there was interposed, immediately following the cold-work and before commencement of prescription hardening heat treatment, the short time heat treatment mentioned in the table. In each of Tests Nos. 3 and 4, the application of the short time heat treatment substantially restored creep life in the piece, and in the case of Test No. 4 improved creep life materially over what it had been before cold-work.

Example 2 Per cent Chromium .20 Cobalt 20 M01ybdenum 6 Aluminium 1 Titanium 2.5

Nickel the balance at 1070 C. This enables the time of treatment to be reduced to a period of between 3 and 10 minutes in the furnace.

According to the present invention, the procedure hereinabove described may be combined with certain procedures described in our 00- pending application Ser. No. 143,812, filed February 11, 1950, which describes a process of manufacturing engineering parts from heat resisting metals, such as nickel-chromium alloys of the class herein described, characterized by, among other things, interrupting the hot-work shaping (i. e., forging, rolling, or stamping) when from 75 to 95 per cent of the desired cross-sectional reduction thereby sought has been accomplished,

According to the present invention the procedure described in the preceding paragraphmay be combined with the procedure described in earlier portions of the present specification.

Example 3 By way of illustration the following is representative of a sequence of steps in which the two procedures are combined.

(a) Forging a cast billet of a nickel-chromium alloy of the class described at from 1050 to 1200 C., the reduction in cross-sectional area of the piece being brought to approximately per centof the reduction which isto be accomplished by forging;

(1)) Giving to the piece, as thus partially forged, a solution heat treatment of 5 to 10 hours at 1050 to 1200 C.;

(c) Forging the piece, at temperatures in the general range 1050.to 1200 C., until the desired cross-sectional reduction by means of forging has been brought substantially to completion and the piece has thereby been readied for machining to final shape and tolerances;

(d) Heating the forged piece for 5 minutes to 1 hour at 850 to 1050 C., this being the grain refining heat treatment;

(e) Machining the forged piece to a finished size;

(1) Giving to the machined piece a short time heat treatment of 3 minutes to 1 hour in a furnace, or electrical induction heating of the machlned surfaces for 30 seconds or more-but not above 1 hour, at 850 to 1100 C.; and

(9) Giving to the piece a precipitation hardening heat treatment for 10 to 20 hours at 700 to 800 C.

V The result of employing the foregoing sequence is to assure that the piece, throughout its entire depth, will have high cree life and fine grain structure, and furthermore that the machined surfaces thereof will have as high a creep strength as they would have had in the absence of machining.

Example 4 Another illustration of a sequence of steps in which the two procedures are combined in a special manner is set forth below. In this case a single heat treatment performs the functions of each of the heat treatment steps which are listed as (d) and (f) in Example 3, and the sequence of steps is as follows:

(a) Forging a cast billet of a nickel-chromium alloy of the class described, at from l050 to 1200 C., the reduction in cross-sectional area of the piece being brought to approximately 85 per cent of the reduction which is to be accomplished by forging;

(1)) Givin to the piece, as thus partially forged, a solution heat treatment of 5 to hours at 1050 to 1200 C.;

(c) Forging the piece, at temperatures in the general range 1050 to 1200 C., until the desired cross-sectional reduction by means of forging has been brought substantially to completion and the piece has thereby been readied for machining to final hape and tolerances;

(d) Machining the forged piece to a finished size;

(6) Heating the machined piece for 10 minutes to 1 hour at 850 to 1050 C.; and

(f) Giving to the piece a precipitation hardening heat treatment for 10 to hours at 700 to 800 C.

In this example the heating mentioned in subparagraph (e) serves a two-fold purpose. In combination with the postponement of part of the forging until after solution heat treatment,

it develops creep strength throughout the entire mass of metal in the piece and at the same time preserves a fine grain structure conducive to fatigue resistance. At the same time, it serves the function of restoring creep life in the metal immediately adjacent to a machined surface thereof, which creep life was materially diminished due to machining.

It will be understood that specific times and I temperatures, as disclosed and claimed herein, are approximate and may be varied to some extent as required by the different characteristics of different alloys of the class described. Furthermore, as in other fields of metal working, shorter exposure to a somewhat higher temperature and longer exposure to a somewhat lower temperature may in some cases produce analogous or equivalent effects, as will be understood by those skilled in the art.

We claim:

1. In the process of producing a machined piece from nickel-chromium alloys of the class described containing a hardening element and capable of bein hardened by precipitation heat treatment, the steps of: reducing a cast billet of the alloy to a shaped piece of approximately the desired final shape by hot-working the same at temperatures of about 1050 to 1250 C.; subjecting the same to solution heat treatment at a temperature of about l050 to 1250 C. for a period of time long enough, about 5 to 10 hours, to produce a thorough solution of the hardening element and distribution thereof in the matrix of the alloy; machining the solution heat treated and cooled piece to the desired final dimensions; subjecting the solution heat treated and machined piece to a short-time heat treatment at a temperature of about 850 to 1100 C. for a period of time, not less than seconds or more than one hour, suilicient to restore to the metal lying within approximately .016 of the machined surface the creep strength lost by machining, and

insufficient to. produce surface corrosion; and subjecting the solution heat treated and machined piece after it has been subjected to said short-time heat treatment to precipitation heat treatment at approximately 700 to 800 C. for from about 10 to 20 hours.

2. The process of claim 1 wherein the step therein described as a short-time heat treatment is carried out at a temperature of approximately 850 to 1050 C.

3. The process of claim 1 wherein the step therein described as a short-time heat treatment is carried out at a temperature of approximately 1050 to 1100 C. for from about 3 to 10 minutes.

4. The process of claim 1 wherein the step therein described as a short-time heat treatment is carried out at a temperature of approximately 1070 C. for from about 3 to 10 minutes.

5. I the process of producing a machined piece from nickel-chromium alloys of the class described containing a hardening element and capable of being hardened by precipitation heat treatment, the steps of: reducing a cast billet of the alloy to a major extent not exceeding about of the total desired hot-work reduction by hot-working the same at temperatures of about 1050 to 1250 C.; subjecting the thus partly reduced billet to solution heat treatment at a temperature of about 1050 to 1250 C. for a period of time long enough, about 5 to 10 hours, to produce a thorough solution of the hardening element and distribution thereof in the matrix of the alloy; completing the hot-work reduction by hot-working the solution heat treated billet at temperatures of about 1050 to 1250 C. until the total desired hot-work reduction has been completed; machining the thus solution heat treated and reduced piece to the desired final dimensions; subjecting the solution heat treated and machined piece to a short-timed heat treatment at a temperature of about 850 to 1100 C. for a period of time, not less than 30 seconds or more than one hour, sufiicient to restore to the metal lying Within approximately .016" of the machined surface the creep strength lost by machining and insufficient to produce surface corrosion; and subjecting the solution heat treated and machined piece after it has been subjected tosaid short-time heat treatment to precipitation heat treatment at approximately 700 C. to 800 C. for from about 10 to 20 hours.

6. The process of claim 5 wherein the step therein described as a short-time heat treatment is carried out at a temperature of approximately 850 to 1050 C.

7. The process of claim 5 wherein the step therein described as a short-time heat treatment is carried out at a temperature of approximately 1050 to 1100 C. for from about 3 to 10 minutes.

8. The process of claim 5 wherein the step therein described as a short-time heat treatment is carried out at a temperature of approximately 1070 C. for from about 3 to 10 minutes.

References Cited in the file of this patent UNITED STATES PATENTS Number Name Date 2,012,890 Miller Aug. 27, 1935 2,173,092 Baker Sept. 19, 1939 2,545,862 Scott Mar. 20, 1951 

1. IN THE PROCESS OF PRODUCING A MACHINED PIECE FROM NICKEL-CHROMIUM ALLOYS OF THE CLASS DESCRIBED CONTAINING A HARDENING ELEMENT AND CAPABLE OF BEING HARDENED BY PRECIPITATION HEAT TREATMENT, THE STEPS OF: REDUCING A CAST BILLET OF THE ALLOY TO A SHAPED PIECE OF APPROXIMATELY THE DESIRED FINAL SHAPE BY HOT-WORKING THE SAME AT TEMPERATURES OF ABOUT 1050* TO 1250* C.; SUBJECTING THE SAME TO SOLUTION HEAT TREATMENT AT A TEMPERATURE OF ABOUT 1050* TO 1250* C. FOR A PERIOD OF TIME LONG ENOUGH, ABOUT 5 TO 10 HOURS, TO PRODUCE A THOROUGH SOLUTION OF THE HARDENING ELEMENT AND DISTRIBUTION THEREOF IN THE MATRIX OF THE ALLOY; MACHINING THE SOLUTION HEAT TREATED AND COOLED PIECE TO THE DESIRED FINAL DIMENSIONS; SUBJECTING THE SOLUTION HEAT TREATED AND MACHINED PIECE TO A SHORT-TIME HEAT TREATMENT AT A TEMPERATURE OF ABOUT 850* TO 1100* C. FOR A PERIOD OF TIME, NOT LESS THAN 30 SECONDS OR MORE THAN ONE HOUR, SUFFICIENT TO RESTORE TO THE METAL LYING WITHIN APPROXIMATELY .016'''' OF THE MACHINED SURFACE THE CREEP STRENGTH LOST BY MACHINING, AND INDUFFICIENT TO PRODUCE SURFACE CORROSION; AND SUBJECTING THE SOLUTION HEAT TREATED AND MACHINED PIECE AFTER IT HAS BEEN SUBJECTED TO SAID SHORT-TIME HEAT TREATMENT TO PRECIPITATION HEAT TREATMENT AT APPROXIMATELY 700* TO 800* C. FOR FROM ABOUT 10 TO 20 HOURS. 